1. Field of the Invention
This invention relates generally to the field of drilling wellbores through the earth. More specifically, the invention relates to systems and methods for acquiring data related to wellbore drilling, characterizing the data according to the particular aspect of drilling being performed during acquisition, and determining the possibility of encountering particular drilling hazards by analyzing the data thus characterized.
2. Background Art
Drilling wellbores through the earth includes “rotary” drilling, in which a drilling rig or similar lifting device suspends a drill string. The drill string turns a drill bit located at one end of the drill string. Equipment forming part of the drilling rig and/or an hydraulically operated motor disposed in the drill string rotate the drill bit. The drilling rig includes lifting equipment which suspends the drill string so as to place a selected axial force on the drill bit as the bit is rotated. The combined axial force and bit rotation causes the bit to gouge, scrape and/or crush the rocks, thereby drilling a wellbore through the rocks.
Typically a drilling rig includes liquid pumps for forcing a drilling fluid called “drilling mud” through the interior of the drill string. The drilling mud is ultimately discharged through nozzles or water courses in the bit. The drilling mud lifts drill cuttings from the wellbore and carries them to the earth's surface for disposition. Other types of rigs may use compressed air as the fluid for lifting cuttings and cooling the bit. The drilling mud also provides hydrostatic pressure to prevent fluids in the pore spaces of the drilled formations from entering the wellbore in an uncontrolled manner (“blowout”), and includes materials which form an impermeable barrier (“mud cake”) to reduce drilling fluid loss into permeable formations in which the hydrostatic pressure inside the wellbore is greater than the fluid pressure in the formation (preventing “lost circulation”).
The process of drilling wellbores through the earth includes a number of different operations performed by the drilling rig and its operating crew other than actively turning and axially pushing the drill bit as described above. It is necessary, for example, to add segments of drill pipe to the drill string in order to be able to deepen the well beyond the end of the length of the drill string. It is also necessary, for example, to change the drill bit from time to time as the drill bit becomes worn and no longer drills through the earth formations efficiently. The foregoing examples are not an exhaustive list of such non-drilling operations performed by a typical drilling rig, but are recited here to explain limitations of prior art drilling data recording and analysis systems.
Drilling data recording and analysis systems known in the art make recordings of measurements made by various sensors on the rig equipment, and in some cases from sensors disposed within the drill string, with respect to time. A record of the position of the drill string within the wellbore is also made with respect to time (a time/depth index). Typically, prior art systems use the recorded data and recorded time/depth index to make a final, single record of rig operation and sensor measurement data with respect to depth, wherein the presented data represent monotonic increase with respect to depth. For example, measurements made by sensors in the drill string performed “while drilling” are typically only presented in the final record for the first time each such sensor passes each depth in the wellbore. Data measured during subsequent movement of particular sensors by particular depth intervals may be omitted from the final record.
As is well known in the art, however, a substantial amount of the time during drilling operations the depth of the wellbore is not, in fact, increasing monotonically, but may include operations in which the drill string, for example, is removed from the wellbore, is moved up and down repeatedly, or remains in a fixed axial position while it is rotated and the drilling fluid is circulated. The rig operations which do not result in monotonically increasing depth with respect to time may incur exposure to drilling hazards such as stuck pipe, blowout or lost drilling fluid (“lost circulation”). Drilling data recording systems known in the art do not make effective use of drilling parameters measured during non drilling operations for the purpose of identifying and mitigating the risk of encountering drilling hazards.
It is also known in the art that certain drilling parameters measured during non-drilling operations, such non drilling operations including, for example, withdrawing the drill string from the wellbore (“tripping out”), inserting the drill string into the wellbore (“tripping in”) and adding a segment of drill pipe to the drill string to enable further drilling (“making a connection”), may change over time due to conditions in the wellbore changing over time. For example, a formation that has a fluid pressure therein substantially lower than the hydrostatic pressure of the wellbore may cause a large amount of “filter cake” (compressed drilling fluid solids) to build up at the wellbore wall. Over time this filter cake may become so thick as to make it difficult to remove the drill string from the wellbore, or may risk the drill string becoming stuck in the wellbore. Drilling parameters which may change over time may include, for example, the amount of force needed to withdraw the drill string from the wellbore, the amount of torque needed to overcome friction in the wellbore and resume rotary drilling after making a connection, and an amount of fluid pressure in the wellbore due to moving the drill string axially along the wellbore (“swab” and “surge” pressures). It is desirable to have a system which records drilling parameters with respect to time, determines wellbore depth of the drill string with respect to time, automatically determines the actual operation performed by the drilling rig and analyzes data with respect to the operation, and provides the wellbore operator and/or drilling rig operator with indications of unsafe conditions in the wellbore as the drilling parameters change over time.